Ziegler-Natta catalysts are generally composed of a catalyst support material and a transition metal component. The transition metal component is typically a substituted group 4-8 transition metal, with titanium, zirconium, chromium or vanadium being commonly used. The transition metal is often provided as a metal halide, such as TiCl4. Ziegler-Natta catalysts are used to effectively promote the high yield polymerization of olefins. In the polymerization of olefins, the catalyst is often used in combination with an organoaluminum cocatalyst.
When used to catalyze the polymerization of propylene, a third component, an electron donor, is often used in the catalyst to control the stereoregularity of the polymer. The electron donor may be incorporated into the catalyst during its synthesis (an internal donor), or it can be added to the polymerization reactor during the polymerization reaction (an external donor). In some polymerization processes, both an internal donor and an external donor may be used. Various aromatic esters, diethers, succinates, alkoxysilanes and hindered amines are examples of compounds that have been used as internal and/or external donors.
Typical external donors are alkoxysilanes, which are used to control the stereospecificity of the polymer during the polymerization process. An indicator of the isotacticity of a polymer is the xylene solubles (XS) content. In the presence of sufficient amounts of silane, every external donor features a specific XS plateau, e.g. XS of 1 wt %. Thus the XS can only be varied by depleting the system of the external donor. Unfortunately, the XS content rises steeply when decreasing the amount of silane in the polymerization and therefore adjustment on the industrial scale is a concern. For example only a slight variation in the silane concentration can cause an XS value of 10 wt % instead of 4 wt %, which can lead to a reactor shut down. Moreover, as worst case scenario a typical catalyst containing phthalate can produce polymer with XS values higher than 20 wt % in the absence of a silane.
One well known support material used in Ziegler-Natta catalysts is MgCl2. The MgCl2 material may be complexed with ethanol (EtOH). In preparing the catalyst, most or all of the EtOH reacts with the transition metal halide, such as TiCl4.
For example, U.S. Pat. No. 4,829,034 to Iiskolan describes a Ziegler-Natta catalyst, and a method for making the catalyst, using a MgCl2-xEtOH support in which x is about 3. In Iiskolan, the support material is first contacted with an internal donor, such as D-i-BP (di-isobutyl-phthalate). The support-D-i-BP complex is then combined with TiCl4 to form the catalyst.
U.S. Pat. No. 6,020,279 to Uwai describes a method for making a Ziegler-Natta catalyst by producing a MgClr-xEtOH support in which x=1.5 to 2.1 and the support has an average particle diameter of 91 μm. The support is combined with a titanium halide, such as TiCl4, and an internal electron donor for 10 minutes to 10 hours at 120° C. to 135° C. in the presence of an aliphatic solvent. As internal donors, esters like di-isobutyl-phthalate (Examples) are preferred.
Due to health, environment and safety concerns in connection with the use of phthalate-containing Ziegler-Natta catalysts for the production of polymers with potential skin or food contact, a second driver to develop new Ziegler-Natta catalysts is the need to provide non-phthalate catalyst versions that produce polymers with an identical or at least very similar performance profile as the currently broadly used phthalate-containing Ziegler-Natta catalysts.
Well known alternatives to Ziegler-Natta catalysts based on phthalates as internal donors are versions where various malonates, succinates or diether compounds are used. Unfortunately, the use of such alternative internal donors results in polymers with fully different performance profiles. As an example and a direct comparison, the use of a phthalate based Ziegler-Natta catalyst leads to polymers with a GPC Polydispersity Index (PI) (also referred to as Molecular Weight Distribution or Mw/Mn) in the range of 6.5 to 8, when using certain diethers as an internal donor the polydispersity is much more narrow (4.5 to 5.5), and when using succinate as internal donor the polydispersity is 10 to 15 (Polypropylene Handbook, 2nd Edition, Editor: Nello Pasquini, Carl Hanser Verlag, Munich, 2005, page 18, Table 2.1 and P. Galli, G. Vecellio, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 396-415 (2004), pages 404-405 and Table 1).
The molecular weight distribution is one of the most important properties of a polymer. By changing this parameter, the crystalline structure and the crystallization rate of a polymer is dramatically influenced, which has impact on the convertability and usability of a polymer for a given application. As an example, for extrusion applications like sheet, pipe, film, raffia, or thermoforming, a broader molecular weight distribution is advantageous, while for applications like fiber or injection molding a narrower molecular weight distribution would be advantageous. As being accustomed to processing polymers produced with phthalate based Ziegler-Natta catalysts, the converters insist in molecular weight distributions typically produced by such catalysts and expect that phthalate free Ziegler-Natta catalysts deliver a similar molecular weight distribution. Unfortunately, state of the art diether based catalysts deliver polymers where the molecular weight distribution is too narrow while succinate based catalysts deliver polymers where the molecular weight distribution is far too broad.
The xylene solubles (XS) content is another very important property of a polymer, and is an indicator for the stereospecificity of a polymer. By changing this parameter, the crystalline structure and the crystallization rate ofa polymer is dramatically influenced as well, which has impact on the usability of a certain polymer for a given application, as stiffness and toughness of polymer resins as well as their behavior during processing, are widely dominated by the content of xylene solubles (XS).
As external donors, alkoxysilanes are broadly used. These compounds regulate the stereospecificity of the polymer and thus the amount of the xylene soluble content (XS) generated in the polymerization. The range of such xylene soluble contents (XS) is typically between about 1 and 6 wt % and depends on the designated application field for the polymer. As an example, in the case of polymers used in the field of film applications, such as biaxially oriented films (BOPP), the XS should be high (3 wt % up to 6 wt %). In the case of certain injection molding applications, the XS content of homo polymer resins or of the homo polymer part of heterophasic impact co-polymers should be as low as possible, preferably lower than 1.5 wt %, most preferably 1 wt % or even lower. Other important grades require XS values between 2 wt % and 3 wt %, such as for use in applications like fiber, raffia, thermoforming and thin wall injection molding. As accustomed to processing polymers which are produced with phthalate based Ziegler-Natta catalysts, the converters insist in xylene soluble contents typically produced by such catalysts and expect that phthalate free Ziegler-Natta catalysts deliver a similar xylene solubles range.
Unfortunately, state of the art diether based catalysts deliver polymers where the xylene solubles content is high, and when external donors like silanes are used to reduce the amount of xylene solubles, the technically possible reduction is low, and as a side effect the catalyst productivity drops dramatically. As a typical example a diether catalyst without addition of an external donor produces a polymer with a xylene soluble content of 4 wt %. Using the same diether catalyst together with an external donor, the xylene soluble content in the polymer can be reduced to about 2 wt %, but at the same time the catalyst productivity is reduced from 30 kg polymer/g catalyst to 15 kg polymer/g catalyst. Xylene solubles of less than 2 wt % and above 4 wt % are out of reach and accordingly, such catalysts can be used for special applications only, but cannot be used as universal catalysts covering the whole xylene soluble range typical of the numerous grades manufactured by a polymer producer. As a consequence, today diether catalysts are niche catalysts and are used for the production of specialty polymers like fiber grades where the combination of a narrow molecular weight distribution in combination with a fixed amount of xylene solubles of about 2.5 wt % is of certain value.